As vitamin E, a natural type of vitamin E, namely d-.alpha.-tocopherol, and synthesized vitamin E such as dl-.alpha.-tocopherol are known, and various kinds of esters thereof, for example, acetic acid ester, nicotinic acid ester, and succinic acid ester, are also well known. In this invention, such vitamin E or ester thereof is called as V-E. In the medical field, various utilities of V-E have been known, for example, a drug for defects in peripheral circulation or a suppressor for increase of lipid peroxide. Further, V-E is also known to be useful for treatment of various eye diseases. Especially, it has been reported that ophthalmics containing V-E in higher concentration could be useful for treatment of cataract (Journal of the Eye, 8, 93 (1991)).
V-E, however, is sparingly soluble in water. Therefore it is necessary to use a surfactant to formulate V-E in an aqueous ophthalmic preparation. As a prior art, a technology to dissolve or emulsify V-E in water by using surfactant such as polysorbate 80, polyoxyethylene hydrogenated castor oil or lecithin has been reported(Japanese Unexamined Patent Publication 106018/87). It is also described in prior art that rather a large amount of surfactant was needed to make an aqueous ophthalmic preparation of V-E.
To prepare multi-use type ophthalmics, which are generally used ophthalmics, a preservative is needed to prevent secondary contamination, while a surfactant may decrease the effect of the preservative.
Therefore, the decrease of the preservative effect becomes a very important problem to prepare aqueous ophthalmics of V-E which needs rather a large amount of surfactant.
Furthermore, if a preservative is formulated in a higher concentration to solve such problem, it may cause defects in eye tissues and an increase of eye irritation.
In conclusion, a study of a preservative is very important to make an aqueous ophthalmic preparation containing V-E.